FIG. 1 illustrates an exemplary drive circuit for a DC motor. In this exemplary embodiment, the DC motor M1 can be connected to a battery by a single pole double throw relay RLY1 the operation of which is controlled by switch S1. When switch S1 is closed, the relay armature contact will move to the normally open contact, connecting the positive lead of the motor to the battery. This will cause a current to flow which will increase from zero to a maximum over a period of time controlled by the motors inductance. The magnitude of the current in amps will be the battery voltage divided by the motors DC resistance and will approach the motors specified “locked rotor” current. As the armature comes up to speed, the current will decrease to the motors running current, which will be a function of the load on the motor, if any.
Start up current for an exemplary motor circuit is shown in FIG. 2. These results were obtained using a motor with a D.C. resistance of 0.3 ohms, an inductance of 100 micro henries and a no load current of 5 amps connected to a 12 volt battery. When switch S1 is opened, the relay armature contact will move back to the normally closed contact which, being connected to the motor positive lead, will put a short circuit across the motor, thereby causing the motor to stop within a few revolutions. At the instance of disconnect from the battery, the motor becomes a generator whose voltage will be that of the battery, and whose instantaneous stored current will be equal to that applied to the motor at start up. This current will be required to flow through the normally closed contacts. FIG. 3 illustrates the current flow through the contacts of a relay the exemplary motor circuit.
Referring to FIG. 4, when power is applied to the relay coil, the armature moves to the normally open contact and is held there by the magnetic force generated. When coil power is released the armature returns to the normally closed contact and is held there by the return spring. The coils magnetic force causes a greater normally open contact pressure than that caused by the return spring tension on the normally closed contact. Because of this, relays generally have a lower maximum switching current specified for the normally closed contacts than for the normally open contacts. The normally closed contact current can range from 50% to 75% of the normally open contact current.
When DC motor braking relay contacts close, the possibility exists of the contacts welding at the instance of contact. Such welding can significantly reduce the life cycle of the relay. Therefore, it is desirable to provide a circuit arrangement that inhibits current flow until after the contacts are fully closed, thereby greatly extending the electrical life of the contacts.